Epitaxial ZnSe Films Research Articles

Formation and reconstruction of Se nanoislands at the surface of thin epitaxial ZnSe layers grown on GaAs substrates

Strained epitaxial ZnSe layers are grown on GaAs substrates by the method of vapor-phase epitaxy from metal-organic compounds. It is found that Se nanoislands with a density of 108 to 109 cm–2 are formed at the surface of such layers. It is established that an increase in the size of Se islands and a decrease in their density take place after completion of growth. Annealing in a H2 atmosphere at a temperature higher than 260°C leads to the disappearance of Se islands and to a decrease in the surface roughness. It is shown that annealing does not lead to deterioration of the structural perfection of the epitaxial ZnSe films; rather, annealing gives rise to a decrease in the intensity of impurity–defect luminescence and to an increase in the intensity of intrinsic radiation near the bottom of the exciton band.

Lineshape analysis of ZnSe longitudinal optical phonons near theE0gap excitation of epitaxialZnSe∕GaAs(001)

Longitudinal optical (LO) phonon lineshape of pulsed laser deposited ZnSe epitaxial films on (001) oriented GaAs with near ${E}_{0}$ gap excitation is investigated. Temperature dependence Raman data with $4579\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ excitation show a shift of resonance from 2LO anti-Stokes to the LO Stokes in the temperature range of $150--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The ZnSe LO lineshape does not fit a Lorentzian profile and is characterized by the presence of large wings on both the lower and higher frequency sides of the LO phonon. The wings on the lower frequency side of the LO phonon are attributed to the Fano interaction of the LO phonon and an electronic continuum due to photogenerated carriers. The electronic continuum is observed in Stokes and anti-Stokes Raman spectra when in resonance. We have proposed an electronic transition scheme based on the presence of donor states below the conduction band to explain the observed electronic continuum. The dependence of the intensity of the continuum with the incident laser intensity is also accounted for in the same scheme. Wings at the higher frequency side of the ZnSe LO phonon are attributed to a combination phonon $[\mathrm{TO}(\mathbit{q})+\mathrm{TA}(\mathbit{q})]$ at $\ensuremath{\sim}270\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, which is also confirmed by the temperature behavior of the mode.

Optical characterization of double layers containing epitaxial ZnSe and ZnTe films

In this paper results of the optical characterization of double layers consisting of ZnTe and ZnSe thin films prepared by molecular beam epitaxy onto GaAs single crystal substrates are presented. For this optical characterization the optical method based on combining variable angle spectroscopic ellipsometry and near-normal spectroscopic reflectometry is used in the multi-sample modification applied within the spectral region 230–850 nm. Using this method the spectral dependences of the optical constants of the upper ZnTe thin films are determined within the spectral region mentioned above. Spectral dependences of the optical constants of the lower ZnSe thin films were determined within the spectral region 450–850 nm. Boundary roughness of these double layers and overlayers is respected. RMS values of the heights of the irregularities of the boundaries and thicknesses and optical constants of the overlayers are determined by means of the combined optical method as well. The uppermost boundaries of the double layers are, moreover, analysed using atomic force microscopy because of verification of the RMS values of these boundaries obtained by the optical method. The spectral dependences of the optical constants of the upper ZnTe films and lower ZnSe films determined in this paper are compared with those presented for ZnTe single layers and ZnSe single layers in the literature.

Electrochemical Deposition of ZnSe from Dimethyl Sulfoxide Solution and Characterization of Epitaxial Growth

In this work, we studied the conditions for the electrodeposition of ZnSe films from dimethyl sulfoxide solutions with a special focus on epitaxial growth. We examined the influence of experimental parameters such as the nature of the substrate, temperature, and hydrodynamic conditions. The investigated substrates were glass covered with SnO2:F, molybdenum and (1̄1̄1̄) or (100) n−InP substrates. The voltammetric behavior of the Zn(II)−Se−DMSO system showed that the mechanism of ZnSe film formation is strongly dependent on the nature of the substrate. Films prepared at constant potential were characterized by scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), atomic force microscopy (AFM), reflection of high energy electron diffraction (RHEED), transmission electron microscopy (TEM) and also by optical measurements. Epitaxial growth of ZnSe was obtained on (100) and (1̄1̄1̄) n−InP crystalline planes with good quality as demonstrated by RHEED patterns and TEM measurements, a result that confirms that electrodeposition provides an alternative route to obtain epitaxial films of ZnSe.

A Novel Approach for the Electrodeposition of Epitaxial Films of ZnSe on (111) and (100) InP Using Dimethylsulfoxide as a Solvent

Results on the preparation and characterization of ZnSe epitaxial thin films obtained by electrodeposition on (111) and (100) n-type InP single-crystalline substrates, starting from elemental selenium dissolved in DMSO at423 K, are reported. Voltammetric studies have been performed on these substrates and compared to SnO 2 :F covered glass substrates. They show substrate dependent behavior and the presence of parasitic faradaic reactions attributed to the formation of di-selenide anions. However, composition analysis of the films by electron dispersive spectroscopy revealed that, independent on the substrate, nearly stoichiometric ZnSe films are obtained within the deposition potential range. Scanning electron microscopy images present a specific effect of chloride ion on the films morphology, whereas their optical characterization showed a direct band gap value of 2.64 eV. The epitaxial quality of the deposits has been confirmed by reflection of high energy electron diffraction and atomic force microscopy techniques.

High-frequency standing longitudinal acoustic resonances in supported thin films

Inelastic light scattering from epitaxial ZnSe films grown on GaAs reveal a large number of near equally spaced phonon modes with characteristic frequencies that depend on the film thickness and scale as the harmonics of a closed-end organ pipe. These periodic excitations that extend well beyond 100 GHz are identified as confined longitudinal acoustic resonances with negligible in-plane wave vector components. The calculated scattering cross sections, which consider the elasto-optic and surface/interface ripple channels in the film and substrate, account for all significant features of the Brillouin spectra. These include the conspicuous appearance of standing wave harmonics only beyond \ensuremath{\sim}45 GHz and apparent absence of Brillouin peaks related to alternate harmonics.

P-Type ZnO Layer Formation by Excimer Laser Doping

A p-type ZnO layer was fabricated by excimer laser doping technique using thermally oxidized ZnO films. Epitaxial ZnO(0001) films were formed by thermal oxidation of epitaxial ZnSe films on Si(111) substrate grown by remote plasma enhanced metal-organic chemical vapor deposition. The p-type ZnO was fabricated by excimer laser irradiation with Sb as a dopant source. A good Ohmic contact was obtained between Sb doped ZnO layer and gold metal electrodes. The Sb doped ZnO layer showed positive Hall coefficient, the resistivity was 8 × 10—3 Ω cm with a hole mobility of 1.5 cm2/Vs and an acceptor concentration of 5 × 1020 cm—3 as p-type, respectively.

Crystalline quality of ZnSe thin films grown on GaAs by pulsed laser deposition in He and Ar ambients

Epitaxial films of ZnSe with a thickness of approximately 1 μm, were deposited on (001) semi-insulating GaAs by pulsed laser deposition using a third harmonic Nd-YAG laser at 10 −6 torr vacuum and at various pressures of He and Ar gases. The best crystalline quality of the ZnSe epilayer was obtained when the deposition was carried out at 10 −4 torr of He. The full width at half maximum of the X-ray diffraction rocking curve of the ZnSe film deposited at 400°C in 10 −4 torr of He was approximately 230 arcsecs. A comparison of the reciprocal lattice space mappings showed that ZnSe epilayers deposited in Ar ambient have a much larger variation of lattice spacing and strain than those grown in vacuum and He ambient. The scanning electron micrographs showed that the epilayers of ZnSe deposited at 10 −4 torr of He have the least particulate density. Possible reasons for the observed improvement in the crystalline quality are discussed.

ZnSe growth by radical assisted MOCVD using hollow cathode plasma

Epitaxial ZnSe films were grown by metal organic chemical vapor deposition (MOCVD) method assisted with hydrogen radicals generated by hollow cathode plasma. The amount of hydrogen (H) radicals generated in a hollow cathode plasma (HCP) was twice than that in an inductively coupled plasma (ICP), which was measured by NO2 gas titration technique. The growth rate of the ZnSe film was increased by a factor of three when H-radicals generated by a r.f.-ICP are introduced into the substrate region. On the other hand, the growth rate was increased by a factor of ten using H-radicals generated by HCP in comparison with normal growth without using H-radicals. The X-ray diffraction studies showed the quality of ZnSe films grown by HCP was almost same to the films grown by ICP. The HCP is very useful H-radical generator for a radical assisted MOCVD growth.

Effect of elastic stresses on the IR spectra of lattice vibrations in epitaxial ZnSe films on (001)GaAs substrates

The lattice IR reflection spectra of epitaxial ZnSe films are studied for different thicknesses on a (001)GaAs substrate. The frequency of the TO mode is found to increase for films with thicknesses of 0.8 and 1.2 µm that exceed the critical value dcr≈0.1 µm for ZnSe/GaAs pairs. The effect is explained by the existence of regions with residual stress in the film.

Investigation on structural properties of ZnSe, CdTe, and ZnTe films grown by ionized cluster beam epitaxy

Epitaxial films of ZnSe, CdTe, and ZnTe were grown, on GaAs(1 0 0) substrates by ionized cluster beam epitaxy (ICBE). Streaky reflective high energy electron diffraction (RHEED) patterns indicated good crystallinity and surface flatness of the epitaxial ZnSe, CdTe, and ZnTe films. It was found that the crystalline quality of the epilayers depended on the kinetic energy of the clusters and the substrate temperature. The minimum X-ray rocking curve peak widths of the ZnSe, CdTe, and ZnTe epilayers were determined to be 160, 630, and 110 arc sec, respectively.

Lateral magnetoresistances of epitaxial ZnSe and CdMnTe thin films measured by the microwave contactless method

A microwave contactless method is developed to measure the magnetoresistances of ZnSe and CdMnTe epitaxial layers grown by molecular beam epitaxy on semi-insulating GaAs substrates. The carrier concentrations and Hall mobilities are derived from the experimental data.

Structural and optical properties of undoped and doped ZnSe/GaAs strained heterostructures

Double-crystal X-ray rocking curve (DCRC), Auger electron spectroscopy (AES), transmission electron microscopy (TEM), photoluminescence (PL), and Raman scattering measurements on undoped and doped ZnSe epitaxial films grown on GaAs (100) substrates by molecular beam epitaxy were performed to investigate the structural and optical properties of the ZnSe films. From the DCRC analyses, the grown layer was found to be a ZnSe epitaxial film with high quality. The results of TEM measurements showed that the misfit dislocations of the nitrogen-doped ZnSe thin films was reduced in comparison with those of the undoped ZnSe films. The PL spectra of the nitrogen-doped ZnSe epilayer was dominated by donor–acceptor pair recombination and by a sequence of its longitudinal optical phonon replicas. Raman spectroscopy measurements showed that there was a lattice mismatch between the ZnSe epitaxial layer and the GaAs substrate and that a plasma–phonon coupling mode existed together with its characteristic longitudinal optical phonons. These results indicate that the ZnSe epitaxial films grown on GaAs hold promise for applications as buffer layers for the growth of Zn 1− x Mg x S y Se 1− y . © 1997 Elsevier Science S.A.

Growth of ZnSe films on GaAs(100) substrate by x-ray-enhanced, vapor-phase epitaxy

Epitaxial films of ZnSe deposited on a GaAs(100) substrate are grown by x-ray-enhanced, vaporphase epitaxy (XEVPE) using a URS-55a source (CuKα emission, λ=1.542 A, P∼1–3 mW/cm2) from powdered raw material in a purified hydrogen flow. The differences in the photoluminescence and exciton reflection spectra are investigated for single-crystal ZnSe films on GaAs(100) at T=4.5 K, which are subjected to compressive strain and are grown by x-ray enhanced and conventional VPE. The results indicate an improvement of the crystallographic structure of the epitaxial layer prepared by XEVPE. This is further corroborated by data from x-ray diffraction analysis. The observed phenomena are attributed to x-ray activated adsorption and desorption processes and to a change in the surface mobility of adsorbed atoms.

Growth of ZnSe thin films by radical assisted MOCVD method

ZnSe epitaxial films were grown by the low pressure metal organic chemical vapor deposition (MOCVD) technique using diethylzinc and selenium hydride as the source materials. When H radicals generated by a rf glow discharge are introduced into the reaction site, the growth rate of the ZnSe film was increased by a factor of 10. The reactivity between source materials and H radicals was investigated for different pressures, substrate temperatures as well as for different concentrations of H radicals and source materials. With the assistance of H radicals, epitaxial growth of ZnSe films on Si substrates was realized at substrate temperatures above 400°C. If the plasma is produced by N 2 gas, ZnSe films could be obtained similar to that of the H 2 plasma environment. This method is seen to be a promising technique in doping nitrogen into ZnSe in producing p-ZnSe films.

Time-resolved RHEED Studies of the Growth of Epitaxial ZnSe Films on GaAs by Pulsed Laser Deposition

AbstractFilm growth consists of two basic processes, deposition and surface relaxation, with opposing effects on the evolution of surface roughness. The pulsed-laser deposition (PLD) growth process has the unique feature of having periods of very high deposition rates on μs time scales followed by periods, on the order of seconds, with only surface relaxation. In this paper we report the first efforts towards exploiting this unique feature to study these two basic processes independently. Thin epitaxial films of ZnSe were grown using PLD on (001) GaAs and 2° miscut (001) GaAs substrates. For growth on both the singular and vicinal surfaces, RHEED patterns taken following growth showed clear, streaky first zone and sharp second zone spots, and well-defined Kikuchi lines; these features are indicative of a smooth growth surface and high quality film. No reconstruction of the growth surface was observed, in contrast to behavior observed in molecular beam epitaxy. Time-resolved RHEED measurements show that a single morphology developed during growth on singular (001) GaAs. However, during growth on miscut (001) GaAs, two morphologies developed, one transitory and one appearing to evolve towards steady state. When growth on the miscut substrate was stopped, recovery of the RHEED signal was observed. The rate of recovery could be attributed to two relaxation processes, as differentiated by their time constants. Potential origins of these observations are discussed.

High crystalline quality ZnSe films grown by pulsed laser deposition

We have grown epitaxial ZnSe films on (001)GaAs substrates at 300 °C by pulsed laser deposition (PLD). Before the growth, thin buffer layers of GaAs are also grown by PLD at 300 °C. While the pattern of reflection high energy electron diffraction (RHEED) of the buffer layers is spotty, the pattern of the ZnSe films subsequently grown is streaky, and shows distinct Kikuchi lines and bands. The x-ray rocking curve width of the films is as narrow as 150 arcsec. Photoluminescence (PL) at 10 K of the films shows free and bound excitons, donor-acceptor pairs (DAP), and is free of any deep level emissions, indicating good crystalline quality of the films. Scanning electron microscopy (SEM) shows that the particulate number density of the films is only about 1 particulate per 400 μm2.

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The acceptor level of phosphorus in vapor phase epitaxial ZnSe films co-doped with phosphorus and lithium was investigated. A new donor-acceptor pair (DAP) emission was observed in the photo-luminescence property. Using DAP analysis, the acceptor level was found to be 0.151eV. It is considered that the acceptor level in ZnSe films co-doped with phosphorus and lithium is due to phosphorus complexes.

Growth of ZnSe films on GaAs by ionized cluster beam epitaxy

Growth of ZnSe single crystal films on GaAs(100) was realized by ionized cluster beam epitaxy (ICBE). With a single evaporizing source of ZnSe, the epilayer of ZnSe was identified to be stoichiometric. Streaky reflective high energy electron diffraction (RHEED) showed the fine crystallinity and surface flatness of the epitaxial ZnSe films. The crystallographic quality of ZnSe films was further evaluated by X-ray diffraction. The minimum full width at half maximum (FWHM) of ZnSe epilayers was determined to be 133 arc sec. The dependence of the FWHM on substrate temperature and acceleration voltage was also studied.

Deep electron states in chlorine-doped ZnSe films grown by molecular beam epitaxy

Deep level transient spectroscopy (DLTS) was used to investigate defect centers in chlorine-doped ZnSe epitaxial films grown by molecular beam epitaxy on (100) n+-GaAs substrates. The resulting carrier concentrations were in the range from 8×1015 to 3.8×1018 cm−3. In low and moderately doped samples two isolated point defects are found, with energy levels at 0.30 and 0.51 eV below the conduction band. The concentration of the dominant trap (at 0.51 eV) is relatively low— of the order of 1015 cm−3—and does not depend on the Cl-doping level. The trap exhibits a strong electric field dependence, indicating its donorlike character. Heavily doped samples reveal a single thermal emission peak. The DLTS amplitude of this peak changes as a logarithm of the filling pulse duration, suggesting that the emission originates from spatially extended defects. We compare the DLTS behavior observed on ZnSe:Cl to earlier studies of Ga-doped ZnSe. Our results clearly indicate that Cl is far superior to Ga as an n-type dopant, because—unlike Ga—Cl does not of itself introduce any detectable deep defects into ZnSe.